1. Technical Field
The present invention relates to coherent waveform based imaging, and more particularly to an optical coherence tomography (OCT) imaging system.
2. Statement of the Related Art
Many imaging systems utilize coherent waveforms to obtain information regarding target objects of interest. Examples include OCT, ultrasound diagnostics, and synthetic aperture radar. OCT is a low-coherence interferometer-based noninvasive medical imaging modality that can provide high-resolution sectional images of biological tissues (see for example, U.S. Pat. No. 5,321,501, U.S. Pat. No. 5,459,570, Huang, D. et al. (1991). “Optical coherence tomography.” Science 254(5035): 1178-81). Since first introduced, OCT has been used in a variety of medical research and diagnostic applications. One successful application of OCT imagery can include the use of OCT in the dermatological imaging of the skin. Another successful application of OCT imagery can include sectional imaging of the retina in the field of ophthalmology. In this regard, time domain based OCT has produced cross-sectional images of the retina of the eye that have proven value to ophthalmologists (see for example, Swanson, E. A. et al. (1993). “In-vivo retinal imaging by optical coherence tomography.” Optics Letters 18(21): 1864-1866; Izatt, J. A. et al. (1993). “Ophthalmic diagnostics using optical coherence tomography”. Ophthalmic Technologies III, SPIE, 1877: 136-144, Los Angeles, Calif., USA). Notwithstanding, time domain OCT instruments cannot acquire sufficient data to characterize completely important retinal pathologies.
The limitations of time domain OCT are the natural result of the inherent difficulties in acquiring and processing imagery of an unstable target—the human eye. For example, although ophthalmic OCT has been commercialized for several years, the spatial registration of an OCT image to fundus landmarks has not been achieved satisfactorily. In this regard, fundus landmarks can be used to relate different structural abnormalities at different retinal locations. Precise spatial registration of OCT sections to tissue location also can be important when interpreting other medical images. Yet, the unavoidable eye movement of a patient during image acquisition can complicate the ability to achieve precise spatial registration due to the unavoidable distortion of the OCT image.
As an alternative to OCT, the scanning laser opthalmoscope (SLO) provides en face fundus images familiar to ophthalmologists (see for example, Sharp, P. F. et al (2004) “The scanning laser ophthalmoscope—a review of its role in bioscience and medicine” Physics in Medicine and Biology 49: 1085-1096). In this regard, en face views are familiar to ophthalmologists not only from direct observations, but also from fundus photographs and fluorescein angiography. The strength of an en face view is that structural abnormalities at different retinal locations can be related to each other and to major retinal landmarks such as the fovea and optic nerve head. In any case, combining OCT with SLO (SLO/OCT) provides one possible means for precise spatial registration of the OCT image while providing an en face fundus image (see for example, U.S. Pat. No. 5,975,697, U.S. Pat. No. 6,769,769, CA2390072, US20040036838, US20040233457, and WO2004102112).
Specifically, time domain SLO/OCT systems utilize two-dimensional transverse scans to provide sectional images in planes perpendicular to the depth of the sample. In an SLO/OCT system, the fundus image can be acquired by splitting the reflected sample light during the transverse scan into two detection channels. A first channel can accommodate OCT while the second channel can be utilized in acquiring intensity image (see for example, U.S. Pat. No. 5,975,697, U.S. Pat. No. 6,769,769, CA2390072, US20040036838, US20040233457, and WO2004102112). As an alternative approach, the sectional images can be summed along the depth of the image (see for example, Hitzenberger, C. K. et al. (2003). “Three-dimensional imaging of the human retina by high-speed optical coherence tomography.” Optics Express 11(21): 2753-2761; Puliafito C. A. “Summary and Significance” American Academy of Ophthalmology Subspecialty Day on retina, Section X: Ocular Imaging, New Orleans, Oct. 23, 2004, 3:06 pm). The approach of two detection channels can require a more complicated setup and the signal-to-noise ratio of the OCT may be reduced by a partial sacrifice of the back-reflected sample light. By comparison, in the approach of summing the OCT images along their depth, accuracy can be sacrificed when the eye moves between different sections.